1. Field of the Invention
This invention relates to an input detection system for detecting the presence and position of an opaque element within an irradiated field and more particularly to a touch input system employing infrared light emitting devices and light detecting devices to form a matrix of infrared beams spanning a display surface such that the presence of an opaque element or stylus will interrupt one or more beams.
2. Description of the Prior Art
The use of video displays as an output for information from a computer can present the operator with the potential to manipulate the displayed information. Traditionally, control of displayed information or cursors has been via a keyboard. Recently, a number of devices have been introduced which allow an operator to directly interact with the video display. These types of devices have included light pens, desk-type mouse controllers, or touch input devices such as a switch matrix or opto-electronic matrices. While generally switch-type overlays, placed adjacent a video display, are inexpensive to apply and utilize, they are generally susceptible to contact wear as well as distortion of the video information which is presented to the viewer, particularly in high usage environments. However, opto-matrix schemes utilize light, which is generally in the infrared region, and the switch matrix presented by the light beams is invisible to the viewer and, therefore, does not distort the video information displayed to the viewer and is not subject to wear in high usage environments. A number of schemes which utilize opto-matrix frames may be found in U.S. Pat. No. 4,267,443, entitled "Photoelectric Input Apparatus", issued May 12, 1981 to Carroll et al.; and U.S. Pat. No. 3,764,813, entitled "Coordinate Detection System", issued Oct. 9, 1973 to Clement et al. These schemes address problems inherent with opto-matrix devices such as increasing frame resolution without a corresponding increase in components, surrounding or ambient light compensation, or optimization of emitter/detector driving and detecting networks, respectively. These systems still have drawbacks. Large component usage results in higher costs. Some systems exhibit difficulty in compensating for reflection or glare resulting in styli hits which are not recorded.
Ambient light also results in significant problems. Several systems for ambient light compensation have been proposed, and some are marketed in touch entry systems. For example, U.S. Pat. No. 4,267,443 discloses an ambient light sampling system. U.S. Pat. No. 3,970,846 discloses an ambient light compensation feedback loop for affecting the bias voltage at the base of a phototransistor.
It is desirable to have a device which minimizes the number of components necessary for addressing and detecting emitters and detectors. It is also desirable that the device dynamically compensates for ambient light and for variations in emitter output and detector sensitivity. At the same time, the device should employ a minimal number of components for addressing and detecting emitters and detectors
Traditionally, ambient light compensation schemes have required the use of a large amount of energy to drive light emitting devices, such as infrared light emitting diodes, because intensity of the light emitted by the diode must be greater than the intensity of the ambient light. In practice, this requirement has meant that expensive light emitting and light detecting devices must be employed. Even high performance light emitting diodes and phototransistors must be operated outside of their normal operating ranges.
There is a need for a touch input system which furnishes good ambient light compensation while permitting the use of low cost, readily available components. The necessity of precisely aligning LED's and phototransistors is also a problem. Further reduction of the intensity of the light emitted by the LED's is also highly desirable. The system disclosed in the instant application permits such improvements.